Various methods are known for obtaining information to evaluate the quality of a resistance welding connection and/or to control or regulate a resistance welding method. For example, WO 03/028936 A2 discloses a device for obtaining information to control a resistance welding method in which measuring equipment is provided to detect the temperature of the welding electrodes and/or the workpiece. The prior-art device is used to detect the temperature of the welding electrodes and/or workpiece during the welding process to gain information on the progress of the welding method. If, for example, measuring the temperature of the welding electrodes indicates that one of the welding electrodes is hotter than the other, one can deduce that a weld nugget that has formed on the contact surface between the workpieces being welded is shifting toward the hotter welding electrode, which is undesirable in certain circumstances. To prevent such a shift and obtain a symmetrical arrangement in relation to the contact surface between the workpieces being welded, the parameters of the welding method can be controlled or regulated as a function of the measured temperatures. The prior art device provides very precise information on the temperature of the welding electrodes; however, a substantial amount of equipment is required.
Similar devices are also known from Patent Abstracts of Japan, vol. 004, no. 154 (M-038), Oct. 28, 1980 & JP 55106693 A, U.S. Pat. No. 4,214,164 A, EP 0 252 624 A, Patent Abstracts of Japan, vol. 005, no. 172 (M-095), Oct. 31, 1981 & JP 56099082 A, WO 00/59673 A, DE 34 16 733 C, CH 411 166 A and U.S. Pat. No. 5,093,553 A.
A method to obtain information to evaluate the quality of a resistance welding connection is also known in DE 43 28 363 C2, DE 43 28 337 C1 and WO 92/10326.
Furthermore, DE 43 38 449 C2 discloses a method to obtain information to evaluate the quality of a resistance welding connection in which, after welding is over, a thermoelectric voltage is measured between one of the welding electrodes and the workpiece that it contacts. A disadvantage of the known method is that it is not suitable to control a resistance welding method during welding.